A vinyl chloride-based polymer is a polymer containing 50% or more of vinyl chloride, wherein it has various applications because it is inexpensive, its hardness is easily controlled, and it is applicable to most processing equipments. In addition, since the vinyl chloride-based polymer may provide a molded article having excellent physical and chemical properties such as mechanical strength, weather resistance, and chemical resistance, the vinyl chloride-based polymer is being widely used in many fields.
The vinyl chloride-based polymer has been prepared by various polymerization methods, and has been mainly prepared by suspension polymerization and emulsion polymerization. The suspension polymerization is a method of preparing a vinyl chloride-based polymer having a particle diameter of 150 μm or less by using water, a vinyl chloride-based monomer, a suspension stabilizer, and a polymerization initiator.
Polymerization for preparing the vinyl chloride-based polymer is an exothermic reaction, wherein, in order to control heat generated during the reaction, heating and cooling jackets are installed to the outside of a polymerization reactor to maintain a constant temperature. However, since there is a limit in controlling the heat generated during the reaction only by the jackets as the polymerization reactor becomes larger to improve productivity, a reflux condenser is installed on an upper portion of the polymerization reactor to removal the heat.
However, since the heat control through the reflux condenser is accompanied by evaporation of a reactant, a foam is inevitably generated by the evaporation. The foam becomes a cause of scale formation, wherein polymer particles incorporated into the foam enter the reflux condenser to not only cause the formation of a scale in the reflux condenser and a reduction in heat removal ability, but to also generate fish-eyes to deteriorate surface properties.
In order to address such limitations, Korean Patent Application Laid-open Publication No. 2013-0025515 discloses a method of preparing a vinyl chloride-based polymer which adjusts a temperature of an upper portion of a reflux condenser included in an overhead of a polymerization reactor to a polymerization temperature of the polymerization reactor and simultaneously adjusts a foam level height in the polymerization reactor to satisfy 70% or less through a non-condensable gas exhaust process. However, in this case, when an excessive amount of the non-condensable gas is exhausted, the foam flows backward to the reflux condenser to form a scale, and, as a result, fish-eyes may occur.
Thus, in order to prepare a vinyl chloride-based polymer with excellent surface properties, there is a need to develop a method of adjusting a foam generated during polymerization to an appropriate level.